Ion channel properties are studied in the excitable membranes of nerve and muscle cells grown in tissue culture. Measurements of single-channel fluctuations are done using an external patch electrode. Such measurements characterize the unit conductance and gating kinetics of the channels. Membrane noise analysis, voltage clamp, and radioactive tracer experiments are used to characterize the gating dynamics, ion selectivity and pharmacological properties of postsynaptic, electrically excitable and immunologically-induced ionic channels.